In response to rapid development of mobile communication and Internet technology and various demands of subscribers, a recent mobile communication service has been developed to provide a high speed packet data service through which it is possible to transmit e-mails or large amounts of digital data such as a still picture as well as to provide voice communication service. Consequently, the mobile communication system progresses from a circuit switched (CS) domain for voice communication to a packet switched domain for packet communication.
Further, a core network for providing a multimedia service through the Internet has been changed from the prior packet switched basis to a packet basis and gradually progresses on the basis of an Internet protocol. The communication system which provides an IP multimedia service (IMS) through an IP based core network to a subscriber is called an IP multimedia subsystem. Various studies for providing all IP service through IMS have progressed in 3GPP (Third Generation Partnership Project)/3GPP2.
As a circuit switched method is used for typical voice and real time service, it means a way of forming a fixed communication route based on a circuit between a user and his counterparty. Because the IMS uses an unfixed communication route based on a packet to increase a transmitting efficiency and ensure stability, the IMS is suitable for a data service, a messaging service and a file transmission service. The IMS is able to support a connection, which the plurality of users participate in, as well as a simple one-to-one telephonic communication. Such a packet switched (PS) domain may be called the IMS domain.
The IMS may include a group of a signaling and a bearer which are related to network elements, and approach a Web-based technique for a voice, a video, message data, and a wireless subscriber. Because the IMS provides the various types of packet data services based on the IP transmission protocol, IMS users can exchange a multimedia type of contents, such as a picture, a video clip, a sound clip, and the like, through session based messages with one another.
As the VCC service is a technique proposed for transferring a voice call between the CS domain and the IMS domain through domain transfer (DT), it is provided to a subscriber subscribed to the VCC service as a home IMS application. A terminal is called a VCC terminal, which can seamlessly change its domain of a call between the CS domain and the IMS domain.
Further, for providing the VCC service, an application server (AS) for the VCC service is provided in the IMS domain. The VCC service is classified into two functions, where the first function is a domain selection for a terminal and the second function is a domain switching. That is, in the VCC service, a voice call request is transferred through the CS domain or the IMS domain to a terminal. The VCC AS selects either the CS domain or the IMS domain corresponding to the terminal, thereby providing an anchoring for the voice call. Furthermore, the VCC AS provides a domain switching such that the voice call service provided through the CS domain is provided through the IMS domain, or the voice call service provided through the IMS domain is provided through the CS domain.
However, it is difficult to cause the VCC AS to select a domain for a voice call service corresponding to a certain terminal. That is, it is difficult to cause the VCC AS to select a domain to successfully provide a voice call service to a terminal. Consequently, there is a problem of decreasing a domain selection completion rate for a voice call service in the VCC AS. Furthermore, since the voice call service is not smoothly provided to a terminal in a mobile communication system, there is a problem of wasting resources for the voice call service.